useEffect demo (the real app behind the video)

npm install
npm run dev            # http://localhost:5173 (counter) · http://localhost:5173/?show=timer (timer)
npm run lint           # the react-hooks lint rule

src/versions/ holds each step of the story (see its README). src/Counter.jsx, src/Timer.jsx and src/main.jsx are whichever version was copied in last (the finished app: Counter v3, Timer v2, StrictMode on).